5-Ethynyluracil (776) inhibits dihydropyrimidine dehydrogenase (DPD), the rate-limiting enzyme in 5-FU metabolism. This study will determine the safety of 776 given alone; the extent of DPD inactivation by 776; the maximum tolerated dose of 5-FU (with and without leucovorin) given with a fixed dose of 776; toxicities of treatment and the pharmacokinetic profile of 776 and of 5-FU in combination with 776.